This invention relates to a method and apparatus for measuring the electrical conductivity of a liquid, or the concentration of a solution, by using electromagnetic induction.
It is known to measure the conductivity of a liquid by electromagnetically inducing an AC current in the liquid and measuring that current by converting it into a voltage signal. When the liquid is a solution the concentration of the solution can be determined from the measured conductivity.
As shown, for example, in U.S. Pat. No. 3,603,873 and JP 54-34632 (Utility Model), it is usual to use a pair of transformers, one called excitation transformer and the other called detection transformer, each of which has an annular core. In an insulated state the two transformers are immersed in the liquid such that the secondary side of the excitation transformer is electromagnetically coupled with the primary side of the detection transformer by a looping path or coil of the liquid which passes through the center holes of the two annular transformers. In other words, a liquid coupling coil intersects the two annular transformers. When an AC voltage is applied to the primary winding of the excitation transformer an AC current is induced in the liquid coupling coil, and the intensity of this current is proportional to the conductivity of the liquid.
In principle the current in the liquid and hence the conductivity of the liquid can be measured by measuring an AC voltage across the secondary winding of the detection transformer. Actually, however, the accuracy of measurement by this method is restricted by the magnetic and some other characteristics of the detection transformer core so that an error of about 1% or more is inevitable.
To reduce the influence of the characteristics of the detection transformer on the accuracy of measurement it is possible to employ a null-balance method (auto-balance method). For this method, a compensation winding is wound on the detection transformer core in a direction chosen so as to negate a magnetic flux produced by the flow of a current in the aforementioned liquid coupling coil, and a variable resistor is connected to the compensation coil. Based on an error signal representative of a current that flows in the secondary winding of the detection transformer, a servomotor is driven to vary the resistance of the variable resistor thereby to vary a current flowing in the compensation winding such that the error signal approaches zero. When the error signal reaches zero the current flowing in the liquid coupling coil can be determined by reading the resistance of the variable resistor.
However, this null-balance method has inherent disadvantages. Since the apparatus has mechanically movable parts it is necessary to use a high-output amplifier, and the apparatus becomes costly and relatively short in service life. When the variable resistor is a wire wound resistor having a movable contact in contact with the wound wire it is impossible to really continuously vary the resistance, so that a limit is placed on the resolution of the measuring apparatus. Furthermore, since the wound wire of the resistor constitutes a coil and acts as an inductance element the aforementioned error signal includes a reactive current with a phase delay of 90 degrees, and therefore the null point becomes obscure. For these reasons errors in measurement reach .+-.3% or more.
Besides, there are some problems about the durability and stability of the sensing part of a conductivity measuring apparatus of the electromagnetic induction type. In conventional apparatus of this type it is usual to confine the two annular transformers in an insulating body which is to be immersed in a liquid to measure the conductivity of the liquid. The body is formed with a cross-sectionally annular chamber in which the two transformers are received in a coaxial and parallel arrangement and has holes to provide the aforementioned liquid coupling coil. The material of the body is selected according to the properties of liquids which are the objects of measurement. Synthetic resins excellent in chemical resistance, such as hard polyvinyl chloride, heat-resistant polyethylene and polytrifluoroethylene are selectively used according to the required degree of heat resistance. A superior material is a copolymer of tetrafluoroethylene and perfluoroalkylvinyl ether (abbriated to PFA) which is excellent in heat resistance and also in moldability and machinability.
However, in the case of measuring the conductivity of a very corrosive liquid such as a hydrofluoric acid solution there is a problem about the durability of the sensor part of the measuring apparatus even though PFA is used as the body material. Although the cross-sectionally annular chamber in the body is airtightly closed the chamber is defined by relatively thin walls, and the highly corrosive vapor of hydrofluoric acid can permeate the relatively thin walls and intrude into the chamber. Therefore, the windings and cores of the two transformers in the chamber and the leads extending from the transformers are gradually corroded, and in about 1 to 3 months the sensing part is liable to exhibit a serious change in its characteristics in most cases by reason of increased contact resistances. In an extreme case the sensing part fails by wire breaking.
Usually the annular cores of the two transformers are made of a magnetically high-permeability alloy and decreasing the iron loss. To secure the annular transformers in the cross-sectionally annular chamber in the body of the sensing part it is usual to make the outer and inner diameters of the transformers such that the respective transformers make tight contact with the radially outer and inner walls of the chamber. However, in our view this manner of securing is not favorable for the stability of the output characteristic of the sensing part.